There is a continuing trend in the semiconductor industry to increase the functionality and performance of integrated circuit devices by increasing the number of circuit components within a given integrated circuit device. While in certain cases this may be accomplished by increasing the size of the integrated circuit device, in most cases this is accomplished by reducing the size and increasing the density of the circuit components.
Lithographic processes play an important role in achieving the above described goals in the manufacture of semiconductor devices. In particular, during the manufacture of these devices lithographic processes are used to pattern substrates, such as, silicon wafers or processed silicon wafers which are wholly or partially covered by metal, silicon dioxide or polysilicon. Typically, a substrate is patterned by coating the substrate with an energy-sensitive material called a resist. Selected portions of the resist are exposed to a form of energy which induces a change in the solubility of the exposed portions in relation to a given developing agent or etchant. The more soluble portions of the resist are then removed by etching (developing) the resist with a wet chemical etchant or by utilizing a dry etching process, e.g., plasma etching or reactive ion etching. The resulting pattern defined in the resist is then transferred into the underlying substrate by, for example, etching or metallizing the substrate through the patterned resist.
The utilization of the aforementioned dry etching processes are particularly desirable since they do not require the immersion of the semiconductor wafer into an etching liquid. The most common dry etch process, generally referred to as "plasma etching", utilizes plasma to etch films on the semiconductor wafer. Another type of dry etching, generally referred to as "dry developing", utilizes a reactant gas to etch resist films from the semiconductor wafer.
In such dry etch processes, it is generally desirable to predict or detect when the desired layer of material associated with the semiconductor wafer has been etched away. In particular, it is desirable to detect when the semiconductor wafer has been etched to a desired level or "endpoint". For example, systems have heretofore been designed which monitor the emission spectra of the plasma during plasma etching. Included among these are mass-spectrometric and optical interferometric techniques. In the former technique, mass spectrometric analysis of, for example, an etching plasma is employed to determine resist etch end point. That is, for example, once a reaction product typical of the substrate is detected in the mass-spectrometric analysis of the plasma, then the interface between the resist and the substrate is assumed to have been reached, and the etching is discontinued. While this technique is useful, the time resolution inherent in the analysis of reaction products, and thus the accuracy in the determination of the etch end point, is limited by the etch rate of the substrate and by the diffusion times of substrate reaction products to a detector. As a consequence, the determination of etch end point can be in error by up to several minutes. At typical plasma etch rates (of about 500 Angstroms per minute), such errors in the end point determination correspond to at least several hundred Angstroms, and often several thousand Angstroms, of the resist film thickness (typically only 1-2 .mu.m thick), which is very a significant fraction of the resist thickness, and thus a very significant error in the determination of etch end point.
With typical optical interferometric techniques, light is shined on the resist undergoing etching, and a portion of the reflected light is detected. In addition, the intensity of the reflected light is recorded. As is known, the intensity of the reflected light oscillates periodically with time (as resist thickness is reduced) because of successive constructive and destructive interferences between light rays reflected from the bottom of the grooves being etched into the resist and light rays reflected from the underlying interface between the resist and the substrate. Etch end point is generally detected by looking for sharp changes in slope, or sharp changes in the oscillation frequency, of the detected signal. However, the slope of the output signal does not always change abruptly at the end point, particularly if the substrate has optical properties similar to those of the resist, i.e., if the index of refraction of the substrate is approximately equal to that of the resist. In this case, the etch end point may also be in error by an equivalent resist thickness of at least several hundred Angstroms, and often as much as several thousand Angstroms.
In addition to the aforementioned problems, these mass-spectrometric and optical interferometric systems are typically complex and require expensive analysis equipment for the operation thereof.
Thus, a continuing need exists for a method and arrangement which accurately and efficiently detects when an etching system etches a semiconductor wafer down to a desired level.